Method of improving the resistance



Patented Aug. 7, 1951 METHOD OF IMPROVING THE RESISTANCE TO CORROSION AND ABRASION OF CER- TAIN COATED ALUMINUM SURFACES Frank Palin Spruance, Jr., Ambler, Pa., assignor to American Chemical Paint Company, Ambler, Pa., a corporation of Delaware No Drawing. Application September 20, 1949,

Serial No. 116,858

2 Claims. 1

This invention relates to the art of coatin aluminum and alloys thereof in which aluminum is the principal ingredient and in the following disclosure, as well as in the appended claims, the word aluminum is intended to be understood as including alloys thereof in which aluminum is the principal or dominant ingredient.

It has been known to those skilled in this art that aluminum surfaces can be improved as to their corrosion resistance and paint holding ability if they are coated by subjecting them to the action of an acid aqueous solution, the essential coating-producing ingredients of which are fluoride ions, dichromate ions and ions from the class of acids consisting of phosphoric and arsenic acids. Coating treatments of this kind are fully described and claimed in my prior U. S. patents, No. 2,438,877 issued March 30, 1948, and No. 2,471,909 issued April 5, 1949, as well as in my copending application No. 38,192 filed on July 12, 1948, now U. S. Patent No. 2,494,910. Another treatment of this nature is disclosed in the U. S. Patent No. 2,472,864 issued on June 14, 1949, to James H. Thirsk and myself. In the several disclosures just mentioned, various methods and materials are described for the coating of aluminum by treating it with acid aqueous solutions of the nature above specified.

The present invention is based upon the discovery that aluminum surfaces which are coated in accordance with the teachings of the foregoing patents and application may have their resistance to corrosion and abrasion substantially improved if the coated surfaces are subsequently treated with certain aqueous solutions to be described below and the principal object of the present invention is to so improve such coated aluminum surfaces. Another important object is to provide a method for treating such coated aluminum surfaces which will permit a much greater tolerance to wider variations in the coating solutions heretofore familiar to those skilled in the art-for example, those disclosed in the above referred to patents and application. A concomitant object of the present invention is to make it possible to practice the prior coating procedures with somewhat less exactitude and care,and therefore with less experienced personnel which, of course, results in a corresponding decrease in the costs of the coating operation. The present invention also has for an object the provision of a method which minimizes the tendency to powder, which has heretofore been characteristic of some of the coatings produced by prior procedures unless the processes are bp lated in accordance with Formula I Grams Sodium fluoride 5 Chromic acid (CrOs) 10 Phosphoric acid, .1 64

Water, to make 1 liter.

A coating may be obtained by immersing the aluminum surface in the foregoing solution or by flowing or spraying the solution upon the work or by any other convenient technique familiar to the art. Of course, the solution must be left in contact with the metal long enough to act thereupon until the coating is produced. The time required to produce the coating will vary, of course, with the temperature and method of applying the solution to the metal. At ordinary living temperatures it may require as much as five to ten minutes to produce an appreciable coating with the above solution when the solution is flowed upon the metal but this time can be substantially reduced, say to as little as one to two minutes or even less by suitably heating the solution and using it in a spray process.

Other representative formulae for various coating solutions of the nature disclosed in the above referred to patents and application are as follows:

Formula II Grams Ammonium acid difiuoride 30 Potassium dichromate 20 Ammonium dihydrogen phosphate Water, to make 1 liter.

Formula III Grams Sodium fluoride 5 Chromic acid (C103) 10 Phosphoric acid, 75% 61 Arsenic acid 2 Water, to make 1 liter.

Formula IV Grams Sodium fluoride 5.0 Chromic acid (ClOa) 10.0 Arsenic acid 92.5 Water, to make 1 liter.

After treatment with any of the typical solu' tions described above or with other coating solutions of a similar nature, the essential coatingproducing ingredients of which are fluoride ions, dichromate ions and ions from the class of acids consisting of phosphoric and arsenic acids, the coated surface may be rinsed with water and then treated in accordance with the present invention, or the method of the present invention may be applied without a prior rinse with water. The solutions used in carrying out the present invention may be applied by dipping the work into the solution or by spraying or flowing the solution over the surface or by means of any other convenient technique in which the solution can be brought into contact with the coated. surface. The length of time in which the coated metal surfaces are in contact with the solution is not critical, and, in fact, may be rather brief, say 20 to 30 seconds or so. Also the temperature of the solution is not critical, although it is better not to use a solution which is hotter than 140 F. and I prefer to use it at approximately 120 F., or lower. After removal from the solution, drying of the surface completes the treatment whether or not preceded by water rinse.

The treatment of the present method makes use of a relatively strong solution of water soluble dichromate or dichromates taken from the class which consists of the alkali-metal dichromates and ammonium dichromate. The concentration of the dichromate ions (CI'zO'I) should lie between 3.5 and 45 grams per liter at a pH of from 5.0 to 6.5. Typical examples of said soluble dichromates are sodium dichromate and ammonium dichromate, and the following are illustrative formulae for such treating solutions:

Formula V Ammonium dichromate, 20 grams Water, to make 1 liter.

Formula VI Sodium dichromate, 30 grams Water, to make 1 liter.

In the two formulae just given, the water is ordinary tap water, containing the customary salts. Had the water used been distilled or deionized, the pH would have been low and would have to be adjusted by addition of acaustic. In some localities the pH of the raw water may be high enough to warrant the addition of an acid to bring the pH within the operating range. However, when the treating solution is in the operating range of 5.0 to 6.5 there are enough other hand, when the concentrations fall below 3.5 grams per liter soluble dichromate expressed as CrzOr, the beneficial effects are greatly diminished.

This application is a continuation-in-part of my earlier application Serial No. 90,562, filed April 29, 1949, now abandoned.

I claim:

1. In the art of coating aluminumto increase its resistance to corrosion and abrasion where the surface of the metal is coated by subjecting it to the action of an acid aqueous solution the essential coating-producing ingredients of which are fluoride ions, dichromate ions and ions from the class of acids consisting of phosphoric and arsenic acids; the method which consists in treating a surface so coated with. an aqueous solution having a pH of from 5.0 to 6.5 and containing as its essential active ingredient a water soluble dichromate taken from the class which consists of the alkali-metal dichromates and ammonium dichromate in a concentration expressed as ClzO'z of from 3.5 to 45 grams per liter.

2. The method according to claim 1 in which the treating solution is maintained at a temperature not to exceed approximately F.

FRANK PALIN SPRUANCE, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. IN THE ART OF COATING ALUMINUM TO INCREASE ITS RESISTANCE TO CORROSION AND ABRASION WHERE THE SURFACE OF THE METAL IS COATED BY SUBJECTING IT TO THE ACTION OF AN ACID AQUEOUS SOLUTION THE ESSENTIAL COATING-PRODUCING INGREDIENTS OF WHICH ARE FLUORIDE IONS, DICHROMATE IONS AND IONS FROM THE CLASS OF ACIDS CONSISTING OF PHOSPHORIC AND ARSENIC ACIDS; THE METHOD WHICH CONSISTS IN TREATING A SURFACE SO COATED WITH AN AQUEOUS SOLUTION HAVING A PH OF FROM 5.0 TO 6.5 AND CONTAINING AS ITS ESSENTIAL ACTIVE INGREDIENT A WATER SOLUBLE DICHROMATE TAKEN FROM THE CLASS WHICH CONSISTS OF THE ALKALI-METAL DICHROMATES AND AMMONIUM DICHROMATE IN A CONCENTRATION EXPRESSED AS CR2O7 OF FROM 3.5 TO 45 GRAMS PER LITER. 